Banisteria, Number 39, pages 34-46 © 2012 Virginia Natural History Society Patterns of Frog and Toad Vocalization in Fairfax County, Virginia Todd A. Tupper1, Tatiana Galitzin, Christine Bozarth Science, Technology, and Business Division Northern Virginia Community College Alexandria Campus 3001 North Beauregard Street Alexandria, Virginia 22311 David M. Lawlor Huntley Meadows Park 3701 Lockheed Boulevard Alexandria, Virginia 22306 ABSTRACT Anuran (frog and toad) call surveys are used to monitor long-term trends in anuran populations but survey efficacy is reduced if peak calling periods are unknown. We estimated peak calling activity for eight species (American Toad, Green Treefrog, Cope‟s Gray Treefrog, American Bullfrog, Green Frog, Pickerel Frog, Southern Leopard Frog, and Spring Peeper) in Fairfax County, Virginia. We identified significant interspecific differences in detection probabilities and days to first detection. Spring Peeper and American Bullfrog were the first and last anurans to initiate calling, respectively. Sampling at least five times during two sampling windows (ca. 27 March-17 April and ca. 15 May-16 July) is needed for long-term anuran monitoring. Minimum threshold temperatures required for vocalization increased as the season progressed, even during conditions that supported chorusing in weeks prior. Surveys should be rotated to avoid temporal biases and not be conducted when temperatures are below minimum thresholds. Key words: anuran, monitoring, calling anuran surveys, vocalization, calling chronology. INTRODUCTION temperature, water temperature (Pellet & Schmidt, 2005; Gooch et al., 2006), and time of year all interact Anuran call surveys (CAS) are widely used to to affect the timing and intensity of anuran calling monitor long-term trends in anuran (frog and toad) activity (see Saenz et al., 2006) and consequently a populations, both at smaller, local scales (Steelman & researcher‟s chances of detecting anuran calls (Shirose Dorcas, 2010; Cook et al., 2011) and larger, statewide et al., 1997). (Weir et al., 2005), regional (Weir et al., 2005), and These environmental factors vary across latitudes national scales (Weir & Mossman, 2005). CAS are and even regionally within latitudes (e.g., from Rhode especially important because changes in anuran calling Island to Cape Cod, Massachusetts); as a result, so does chronology could be a possible first indication of a anuran calling activity (Berven, 1982; de Solla et al., biotic response to climate change (Gibbs & Breisch, 2006; Tupper et al., 2007; Cook et al., 2011). 2001). CAS are also used for species-specific Therefore, implementing a precise and efficient ecological studies (Tupper & Cook, 2008) and to assess localized, long-term monitoring program can be the effectiveness of habitat restoration efforts (Stevens challenging because it is most effective to design et al., 2002). Environmental factors such as rainfall, air programs around peak calling activities, i.e., when chances of detecting vocalizations are highest (Crouch 1Corresponding author: [email protected] & Paton, 2002; Cook et al., 2011). TUPPER ET AL.: PATTERNS OF FROG AND TOAD VOCALIZATION 35 Although studies have addressed anuran monitoring Site Selection in southern New England (Crouch & Paton, 2002; Tupper et al., 2007; Cook et al., 2011) and the south We used a stratified-random scheme to select 15 Atlantic states (see Bridges & Dorcas, 2000; Dorcas et wetlands for CAS sampling. Using Google Earth al., 2009; Steelman & Dorcas, 2010), to our knowledge version 6 (http://www.google.com/earth/index.html) at systematically collected, non-anecdotal data are limited an „eye altitude‟ of 4.19 km, we created a grid for the mid-Atlantic states (but see Weir et al., 2005; consisting of 122 220 m x 282 m cells over high- Brander et al., 2007). Though various works describe resolution satellite imagery of Huntley Meadows Park. anuran breeding activities (Lee, 1973; Mitchell, 1979; We assigned each cell into one of our strata (northern, Ernst et al., 1997) in northern Virginia and adjacent central, and southern regions of the park) and randomly areas, to the best of our knowledge none have selected five cells (> 200 m apart) in each region. We quantitatively approached this issue for the purpose of then sampled the wetland nearest the center of each optimizing long-term monitoring programs. We present selected cell using CAS methodology. We assigned data collected from two seasons of CAS sampling at each wetland to one of three calling survey routes (one Huntley Meadows Park in Fairfax County, Virginia, route per aforementioned strata), each consisting of five that can be used to help guide implementation of long- wetlands (Fig 1). Selected study wetlands ranged from term anuran monitoring programs in the mid-Atlantic short-hydroperiod, fishless ephemeral wetlands, to states. Our goals were to (1) identify the most permanently inundated wetlands containing fish. appropriate times of year and night (peak detection periods) and corresponding ambient temperatures to Data Collection sample for anuran species with CAS; (2) to identify the number of sampling occasions needed during a species‟ We recorded up to four ordinal calling index values peak detection period to achieve a 90% probability of at (0-3) following North American Amphibian Monitoring least one detection at an occupied site and (3) to Program (NAAMP) guidelines (Weir & Mossman, calculate the number of wetlands needed to sample to 2005) to quantify anuran calling activity where 0 = no estimate occupancy with a standard error (SE) of 0.10 calls, 1 = calling but no overlap between calls, 2 = (Cook et al., 2011). intermediate overlap and 3 = continuously overlapping calls. Sites were typically sampled between 30-min MATERIALS AND METHODS after sunset and 2400 h (Weir & Mossman, 2005). The order in which sites were sampled was rotated to avoid Study Area temporal sampling biases. Because it is well known that detection probability is greatly affected by air and Huntley Meadows Park (Fairfax County, Virginia; surface water temperatures (Gooch et al., 2006; 38°45′20.95″N; 77°06′29.26″W) is a 577 ha park that is predominantly surrounded by densely populated suburban developments, except for a fragmented green corridor on the southeast side of the park (Fig. 1). The majority of the park (the central wetland) is within wet lowland formed by an early meander of the Potomac River. The central wetland is hydrologically connected to the majority of wetlands within the park. However, the vegetational communities existing in the different regions of the park range from early-successional wetlands to later-successional hardwood swamps. Consequently, the abiotic features of these wetlands are also quite different. Water can be tannin-lignin rich, cool, and acidic, to clearer, warm, and more neutral (DL, unpubl. data). All park wetlands are freshwater, and together with the biotic characteristics of the environment, form a biodiverse and important Fig. 1. Huntley Meadows Park, Fairfax County, Virginia ecosystem in Fairfax County (http://www.fairfax (38°45′20.95″N; 77°06′29.26″W). Black and white circles in county.gov/parks/huntley/). enlarged area represent calling survey points. 36 BANISTERIA NO. 39, 2012 Steelman & Dorcas, 2010; Cook et al., 2011) and that signed-rank tests. Paired-samples t-tests were used to both air and water temperatures are good predictors of examine differences in annual rainfall and temperature. anuran body temperatures (Fouquette, 1980), we Normality was assessed with normal probability plots recorded ambient temperatures during each sampling and Kolmogorov-Smirnov tests and equality of event. We placed a thermometer approximately 1.5 m variances was assessed with Levene‟s test for equality above the ground for 5 min to measure air temperature of variance (Zar, 1999). (° ) and we placed a thermometer between 1.5 and 3 cm beneath the water‟s surface for 5 min to measure Determining a Sampling Regime surface water temperature (˚C). In accordance with NAAMP guidelines (Weir & Mossman, 2005), we also MacKenzie & Royle (2005) define an “optimal” recorded sky conditions, noise disturbance, and wind sampling scheme as one that provides an 85% to 95% codes (on the Beaufort scale). probability of confirming that a target species occupies a site. Thus, for each species, we estimated the number Nightly and Seasonal Calling Chronology of sampling occasions per site needed to achieve 90% probability of detecting the target species (see Cook et We were interested in determining when, within al., 2011) at least once during its peak calling period in established NAAMP CAS sampling guidelines (ca. 30 a given year at occupied sites using the formula p* = 1 - min after sunset to ca. 2400 h), detected species were (1 - p)k, where p = maximum naïve detection encountered while chorusing so that future sampling probability and k = number of sampling occasions/site could accommodate known peak periods of nightly (adapted from MacKenzie & Royle, 2005). We activity. We accomplished this by calculating mean, calculated the number of wetlands necessary to sample 95% confidence interval (CI), and range of time to estimate future occupancy (Ψ) rates (α = 0.10) with (minutes) after sunset that chorusing and non-chorusing equation 6.3 in MacKenzie et al. (2006). events occurred for each species. We examined Microsoft Excel 2007 was used to create figures, differences in timing of calling between chorusing calculate equations presented in Mackenzie et al.